OPO's have been known for many years and are widely used for generating coherent radiation in the near/mid IR region around 1-5 μm.
One example of a tunable OPO is described by Walter Bosenberg et al. in “Continuous-wave singly resonant optical parametric oscillator based on periodically poled LiNbO3”, Optics Letters, vol. 21, no. 10, May 15, 1996 (Bosenberg). By using a Nd:YAG pump laser emitting at 1064 nm, Bosenberg demonstrated an OPO based on periodically poled LiNbO3 that could be tuned over 1.54-1.61 μm (signal) and 3.41-3.14 μm (idler) by varying the temperature of the periodically poled LiNbO3 over 110-220° C.
Prior art OPO's have been limited by narrow tuning range, complex construction and/or poor spectral quality.